In motor vehicles, in particular vehicles driven by an on-board engine, which have active and/or semi-active wheel suspension arrangements by means of which the ground clearance of the vehicle, or the distance between the underside of the vehicle and the ground on which it is disposed, is variably adjustable, it is necessary to determine the level of the vehicle, in order to achieve correct geometrical wheel position values. The level of the vehicle is defined by the difference in height of a part of the vehicle which is supported in sprung relationship on a wheel of the vehicle, for example the chassis of the vehicle, relative to an unsprung part of the vehicle, for example a suspension member which is directly connected to the respective wheel.
Reference will be made at this point to FIGS. 1 and 2 of the accompanying drawings to illustrate this point in connection with a wheel suspension arrangement for a motor vehicle wheel.
Referring therefore to FIGS. 1 and 2, reference numeral 14 identifies an unsprung suspension member 14 of the wheel suspension assembly, which supports a wheel hub assembly 16. An upper strut or suspension link 15 and a lower strut or suspension link 13 of the wheel suspension arrangement are pivotally connected to the suspension member 14. Reference numeral 12 identifies a sprung part of the vehicle, for example the chassis structure thereof, which is supported by the two struts or suspension links 13 and 15, by virtue of the latter being pivotally connected to the part 12 of the vehicle, with the suitable interposition of springs, dampers and the like (not shown), on the suspension member 14 which carries the wheel hub assembly 16. The level of the vehicle which is indicated at a in FIG. 1 is defined by the difference in height (h1-h2) of a pivot mounting point 6 for the sprung part 12 of the vehicle to the lower suspension link 13 and a pivot mounting point 7 of the lower suspension link 13 to the unsprung suspension member 14, in each case in relation to the ground 17 on which the wheel 1 is supported. When the wheel suspension arrangement and the wheel 1 are loaded with the normal weight of the spring-suspended part 12 of the vehicle, the components of the wheel suspension arrangement and the wheel 1 occupy the position shown in FIG. 1.
If the vehicle and in particular the chassis thereof is loaded with an additional weight, the components of the wheel suspension arrangement and the wheel 1 assume the positions shown in FIG. 2, by virtue of a downed movement of the chassis relative to the ground 17 on which the wheel 1 is supported. It will be seen that not only does that cause a change in the level of the vehicle as defined by the height difference h1-h2, but that there is also a change in the position of the wheel 1, for example the camber angle c.
The wheel suspension arrangement is intended to be designed in such a way that the wheel enjoys good contact with the surface on which it is disposed, under all possible driving conditions, so that the desired road-holding qualities of the vehicle are maintained. In order to ensure that, the wheel position must adapt to the movements of the wheel suspension arrangement, in regard to all the wheel position parameters (tracking, camber, trail or caster and steering swivel inclination). That is effected on the basis of certain laws which are established by virtue of the dimensions and the arrangement of the kinematic components of the wheel suspension arrangement. Depending on the respective requirements which the vehicle manufacturer seeks to fulfil in terms of the road-holding qualities of the vehicle, the wheel suspension arrangements of different vehicles differ greatly from each other in regard to their geometrical data. In most cases, the geometrical data involved are not known to the workshops which have to carry out maintenance operations on such vehicles. Even if the geometrical data in question are known, the geometrical performance of the wheel suspension arrangement, as a function of different levels of the vehicle, in consideration of loading and other factors, is not known. Accordingly the vehicle manufacturers provide correlation tables which contain the dependency of the wheel positions, more particularly wheel angle positions, on the level of the wheel suspension arrangement, and also set out reference values at which wheel positions and in particular angular positions are to be measured.
Besides the foregoing problems there is the point that the operation of ascertaining the level of the suspension arrangement, which involves measuring the spacing of the wheel suspension arrangement from the ground on which the wheel is disposed, gives rise to difficulties in ordinary workshops, in particular as regards the accuracy which can be achieved in such a measuring operation. In general the position in respect of height of the ground or floor surface on which the wheel is supported cannot be accurately ascertained in an inspection pit as inspection pits are generally provided with upwardly projecting safety edges which constitute an impediment to accurate measurement. It is therefore generally necessary to rely on notional horizontal planes for carrying out the measurement operations. There is therefore not a standardised measuring method and the individual workshops manage with such difficulties, as best they can.